As the operating frequencies used to transmit digital signals across circuits increases, the signal integrity of the transmission signal becomes more important. In particular, transmission signal integrity issues become more important at operating frequencies in the gigahertz frequencies and higher.
Referring to FIG. 1, transmission signals may be propagated on a transmission signal trace 105 within a circuit having a reference plane 110. An electric field 130 and a magnetic field 135 are created when current passes through the transmission signal trace 105. The illustrated electric field 130 and magnetic field 135 are representative of electromagnetic fields that may exist around the transmission signal trace 105. Specifically, the electric field 130 exists within a dielectric layer (not shown) between the transmission signal trace 105 and the reference ground plane 110. The magnetic field 135 exists around the transmission signal trace 105.
Transmitting signals on a transmission signal trace at higher frequencies is complicated by the relative ease with which noise and other interference may distort the transmission signal. Impedance discontinuities are one source of distortion that may degrade the quality of a transmission signal on a transmission signal trace. An impedance discontinuity, as used herein, is a variation in impedance (resistance and reactance) along a transmission signal trace that results in a distortion of the transmission signal at the location of the impedance discontinuity. An impedance discontinuity also may result in a loss of transmission power of the transmission signal.
The impedance of a transmission signal trace may depend on a variety of factors, including trace length, trace thickness, trace width, dielectric layer material properties, and so forth. An impedance discontinuity may occur where the transmission signal trace properties vary. For example, as shown in FIG. 2a, an impedance discontinuity may occur at a geometric, or physical, discontinuity (e.g., bend or taper) on the transmission signal trace 205. A fringing electric field 215 may result at the impedance discontinuity when a current is applied to the transmission signal trace 205.
FIG. 2b depicts a cross-sectional view of the electric field 230, including the fringing electric field 215, that exists between the transmission signal trace 205 and the reference plane 210. The fringing electric field 215 exists outside of the region directly between the transmission signal trace 205 and the reference plane 210. In particular, the fringing electric field 215 is more widely distributed than the representative electric field 130 shown in FIG. 1. It should be noted that even if there is perfect impedance matching in the transmission signal trace 105 of FIG. 1, some fringing fields might still be present. However, there may be more fringing fields in the presence of an impedance discontinuity, as illustrated in FIG. 2a. As stated above, this fringing electric field 215 results from the impedance discontinuity in the transmission signal trace 205 and acts to distort the transmission signal and reduce the transmission power of the transmission signal on the transmission signal trace 205. Furthermore, this fringing electric field 215 and a corresponding distorted magnetic field (not shown) may cause interference in the form of cross-talk on other nearby transmission signal traces (not shown).
Conventionally, impedance matching on a transmission signal trace may be accomplished through one or more techniques that employ empirical adjustment of the transmission signal trace parameters. For example, the transmission signal trace may incorporate design variations of width, thickness, and so forth, which are calculated to compensate for other impedance discontinuities. However, many of the physical attributes of a transmission signal trace may be predetermined in designing the overall circuit. For example, the routing and bends of the transmission signal trace may be predetermined according to overriding circuit design considerations.
As mentioned above, cross-talk interference may occur between two transmission signal traces. For example, a transmission signal on one of the transmission signal traces may cause noise on an adjacent transmission signal trace through electromagnetic coupling. One method of preventing such cross-talk is discussed in U.S. Pat. No. 6,531,932, to Govind et al. (hereinafter “Govind”), which provides noise shielding between signal traces by alternately interspersing guard traces between adjacent signal traces. Because the presence of the guard traces along the length of the signal traces affects the impedance of the signal traces, Govind addresses adjusting the widths of the signal traces to provide impedance matching.
One problem with the method discussed in Govind is that it does not address the possibility of various types of impedance discontinuities, such as bends that cause fringing electric fields, which are not affected by the disclosed guard traces. Furthermore, the noise shielding techniques in Govind fail to solve the problems presented when the physical attributes of the signal traces have already been established. Another problem with the method of Govind is that it places guard traces along substantially the entire length of the traces and adjust the widths of the signal traces. Such a design method may negatively impact other design parameters, including trace routing, overall circuit size, and cost.